The development and metabolism of mouse CNS is being studied in organotypic cultures. The non-metabolizable amino acid ACPC (1-aminocyclopentane 1-carboxylic acid) produces degeneration of growing or established cultures. Amino acid supplementation of the medium protects cultures against ACPC toxicity, and the effects of individual amino acids are now under study. When goldfish learn a new task or are given intraventricular injections of cyclic AMP, their brains synthesize RN in altered proportions, i.e., more messenger RNA relative to ribosomal RNA than at other times. This effect of cyclic AMP is reprodued in cultures of mouse cerebellum. Noradrenalin and acetylcholine also increase the proportion of messenger RNA synthesized, but cyclic GMP inhibits total RNA synthesis and decreases the proportion of messenger RNA. Other neurohumors and cellular messengers are under study. The culture method is being used to study the neurological mutant disease quaking. Quaking cultures form no myelin, but develop normally in all other respects: thus, the disease is reproduced in culture and must be intrinsic to the affected tissues. Possible interactions between quaking and normal explants in the same culture vessel are being investigated. Scanning elctron microscopy is being used to study the surfaces of growing nerve fibers and migrating cells during development and myelination, in mammalian brain cultures and in the transparent tip of the tail of Xenopus laevis tadpoles.